1. Field of the Invention
The present invention relates to a magnetooptical recording medium with overwrite capabilities and a magnetooptical reproduction method for using such a magnetooptical recording medium. More specifically, the present invention relates to a process for writing over existing or previously recorded information on a magnetooptical recording medium by irradiating the medium while modulating pulses of a laser beam in accordance with the information to be recorded thereon, without modulating the orientation or strength of the magnetic recording bias field Hb.
As used herein, the term "overwrite" refers to the action of recording new information on a recording medium without first erasing existing or previously recorded information. Thus, it is necessary that the previous information not be reproduced. More specifically, the term "overwrite" as used herein refers to the described process of writing over the previously recorded information on a magnetooptical recording medium.
2. Description of Related Art
Recently, there has been a great effort to develop optical recording and reproduction methods, as well as recording devices, reproduction devices and recording media that can be used with these methods, which satisfy the need for high density, large capacity, high access speed and high recording and reproduction speeds. In particular, among the wide range of optical recording and reproduction methods, the magnetooptical recording and reproduction method has the most appeal because of the unique benefit of the method wherein information that is recorded on a magnetooptical recording medium can be erased and new information recorded thereon. Additionally, the process may be repeated many times, thus allowing the magnetooptical recording medium to be reused.
A magnetooptical recording medium used in a magnetooptical recording and reproduction method has a magnetic layer or layers as a recording layer, generally applied on a protective substrate. The magnetic layer comprises, for example, amorphous GdFe, GdCo, GdFeCo, TbFe, TbCo, TbFeCo, and the like. Concentric or spiral tracks are formed on the recording layer, and data is recorded on the tracks. In the present specification, either the orientation of the magnetic field of "upward" or "downward" with respect to the surface of the recording layer is defined as the "A direction," with the opposite orientation becoming the "non-A direction."
Information to be recorded on the magnetooptical disk is digitized beforehand, and the information is recorded using two signals, namely the mark B1, which is magnetized in the A direction, and a mark B0, which is magnetized in the non-A direction. One of the marks B1 or B0 corresponds to the digital signal 1, while the other corresponds to the digital signal 0. In general recording media of the prior art, however, the magnetization of the tracks used for recording information is uniformly set to the "non-A direction" by impressing a powerful external magnetic field on the disk prior to recording. This action of uniformly setting the direction of magnetization is referred to in the art as "initializing" the disk (as the term is traditionally used). Because of this, information is recorded on the disk by creating a mark B1 having magnetization in the "A direction." Information is expressed by the presence or absence of a mark B1, the position of the mark itself, the position of the leading edge and trailing edge of the mark, and the length of the mark. In particular, the method wherein the position of the edge of the mark expresses the information is known as mark length recording. These marks were previously referred to in the art as "pits" or "bits," but are now referred to as "marks."
However, in order to reuse a medium on which recording has already been accomplished, either (1) the medium must be re-initialized (in the traditional sense of the word) using an initializing device to uniformly set the direction of magnetization of the disk; (2) an erasing head similar to the recording head must be provided on the recording device; or, (3) as a prior process, the previously recorded information must be erased using either a recording device or an erasing device. Accordingly, it has previously been impossible to overwrite using a magnetooptical recording medium wherein new information is recorded on a disk regardless of whether any previously recorded information is present.
Such overwriting may be possible if the orientation of the magnetic recording field Hb could be freely modulated between the "A direction" and the "non-A direction" as necessary. However, it is practically impossible to modulate the orientation of the magnetic recording field Hb at the required high speeds. For example, in the case where the magnetic recording field Hb is a permanent magnet, it would be necessary to mechanically reverse the orientation of the magnet such as by physically turning the magnet to change its polarity. However, it is impossible to reverse the orientation of the magnet at the required high speeds for practical recording operations. Even in the case where the magnetic recording field Hb is an electromagnet, it is impossible to modulate the orientation of such large amounts of electrical current at such a high rate.
However, with the phenomenal recent advances in technology, a magnetooptical recording method, a magnetooptical recording medium, and a recording device have been developed to provide overwrite capabilities. The overwrite capable recording method, recording medium and recording device are described in Japanese unexamined patent publication no. Sho 62-175948, German patent publication no. DE 3,619,618 A1, and U.S. Pat. No. 5,239,524, the entire disclosures of which are incorporated herein by reference. In the magnetooptical recording method, the strength of an irradiating light beam is modulated according to the digitized information being recorded without modulating the strength or orientation of the magnetic recording field Hb (including ON and OFF). The references also describe a magnetooptical recording medium with overwrite capabilities to be used with the recording method, as well as a recording device with overwrite capabilities. Hereinafter, these devices will be referred to as the "basic device."
The basic devices advantageously use a multilayered magnetooptical recording medium which has overwrite capabilities. The recording medium includes a recording and reproduction layer (referred to herein as the memory layer or the M layer) and a reference layer (referred to herein as the writing layer or the W layer). The M layer is composed of a magnetic film possessing perpendicular magnetization characteristics, whereas the W layer is composed of a magnetic film possessing perpendicular magnetization characteristics. The two layers are also exchange-coupled. At room temperature, the magnetic orientation of the M layer cannot be changed and only the magnetization of the W layer can be changed to a given specific orientation.
Furthermore, information is recorded and expressed on the magnetooptical recording medium (or disk) by forming marks having an "A direction" (magnetization) and marks having a "non-A direction" (magnetization) in the M layer. In certain cases, the marks can also be formed in the W layer.
With the described recording medium, the magnetic orientation of the W layer can be uniformly set to the "A direction" by an external means, for example, by an initializing reference magnetic field H.sub.ini. Moreover, when the magnetic orientation of the W layer is established, the magnetic orientation of the M layer is not reversed. Furthermore, once the magnetic orientation of the W layer is uniformly set to the "A direction," it is not reversed by the exchange-coupling force from the M layer. Conversely, the magnetic orientation of the M layer is not reversed by the exchange-coupling force of the W layer, which has been uniformly set to the "A direction." As a further characteristic of the M and W layers, the W layer has a weaker coercivity force H.sub.C and a higher Curie point T.sub.C as compared to the W layer.
For use in the described magnetooptical recording method, the magnetooptical recording medium is designed such that prior to recording information thereon, only the magnetic orientation of the W layer is uniformly set to the "A direction" by an external means. This action of magnetically orienting only the W layer is referred to in this specification as "initializing" the disk. However, it is noted that the present use of the term "initialization" is different from the earlier described initialization process, because initialization of a recording media with overwrite capability is different from initialization of other media.
Recording on the recording medium may be conducted by using, for example, a pulsed laser beam. For example, a laser beam with pulses modulated in accordance with the digitized information may be used to irradiate the medium. The strength of the laser beam generally varies between a high level (P.sub.H) and a low level (P.sub.L), thus creating a high level and low level pulse. The strength of the low level pulse is higher than the reproduction level (P.sub.R) with which the medium is irradiated during reproduction. As is already commonly known, the laser beam irradiates the medium at an "extremely low level" even when not recording in order to access, for example, a certain recording location on the medium. This "extremely low level" may be the same as or similar to the reproduction level P.sub.R.
For example, when a medium that has been initialized to the "A direction" receives irradiation from a laser beam at low level P.sub.L, the temperature of the medium rises and the coercivity force H.sub.C1 of the M layer becomes very small, or in the extreme, becomes zero when the temperature of the medium rises above the Curie point T.sub.C1 of the M layer. At this time, the coercivity force H.sub.C2 of the W layer is sufficiently large, so the magnetic orientation of the layer is not reversed by the "non-A direction" magnetic recording field Hb. Furthermore, the force of the W layer affects the M layer through an exchange-coupling force.
The M layer and the W layer are generally composed of alloys of heavy rare earth metals (hereinafter abbreviated RE) and transition metals (hereinafter abbreviated TM). The exchange-coupling force is generally composed of a force that uniformly sets the RE magnetic moments of the two layers, and a force that uniformly sets the TM magnetic moments of the two layers. In the RE-TM alloy, the RE sub-lattice magnetization and the TM sublattice magnetization have opposite orientations, so the orientation of the larger sub-lattice magnetization determines the magnetic orientation of the alloy. When the two sub-lattice magnetizations are equivalent, the composition is referred to as the compensation composition, and the temperature is referred to as the compensation temperature. At temperatures above the compensation temperature, the TM sub-lattice magnetization is stronger; while at temperatures below the compensation temperature, the RE sub-lattice magnetization is stronger.
There are two states for marks prior to irradiation of the medium with a laser beam, namely (1) the state wherein an interface magnetic wall exists between the M layer and the W layer, and (2) the state wherein such a wall does not exist. Marks according to those formed in state (2) are preferred, because marks in state (1) wherein an interface magnetic wall exists do not match marks formed by a laser beam at low level P.sub.L. However, marks in state (2) wherein an interface magnetic wall does not exist match marks formed by a laser beam at low level P.sub.L. In the former case (1), the temperature of the M layer rises under irradiation by a P.sub.L beam. Consequently, the coercivity force HCl of the M layer becomes very small. At the same time, as a result of the effect on the M layer of the force of the W layer via the exchange-coupling force, the magnetization of the M layer, which has a very small coercivity force H.sub.C1, is oriented to a preset orientation (for example, the "A direction") supported by the W layer. As a result, a mark for the state wherein an interface magnetic wall does not exist between the M layer and the W layer (the desired mark) may be formed even in state (1).
Even when the temperature of the M layer temporarily rises slightly higher (above T.sub.C1) and the magnetization of the M layer becomes zero, desired mark formation is not lost. When irradiation by the laser beam ends, the temperature of the recording medium naturally drops and gradually falls to a value below the Curie point T.sub.C1. When this occurs, the magnetization of the M layer reappears. At the same time, the force of the W layer has the same effect on the M layer via the exchange-coupling force. Consequently, the magnetization that appears in the M layer is oriented to a preset orientation (for instance, the "A direction") supported by the W layer. When the recording medium returns to room temperature from this condition, the preset orientation is preserved. However, when the compensation temperature is passed in either the M layer or the W layer during the return to room temperature, the orientation of the magnetization of the respective layer is reversed. This process provides the same marks as in the latter case (2) described above, and is known as the low temperature process or the low temperature cycle.
On the other hand, when a medium initialized for example to the "A direction" is irradiated by a laser beam at a high level P.sub.H, the temperature of the medium rises, the coercivity force H.sub.C1 of the M layer becomes zero, and the coercivity force H.sub.C2 of the W layer becomes very small, or in the extreme, becomes zero. Consequently, the magnetization of the W layer, which has a very small coercivity force H.sub.C2, is oriented to a preset orientation (for example, the "non-A direction") by the magnetic recording field Hb. Even in cases where the temperature of the W layer temporarily rises slightly further and the magnetization of the W layer becomes zero, desired mark formation is not lost. When irradiation by the laser beam ends and the temperature of the recording medium naturally drops and gradually falls below the Curie point T.sub.C2, the magnetization of the W layer reappears. At this time, the magnetization of the W layer is oriented in the same way to a preset orientation (for example, the "non-A direction") under the magnetic recording field Hb. As the recording medium further cools and gradually drops below the Curie point T.sub.C1, the magnetization of the M layer also reappears. At this time, the force of the W layer has an effect on the M layer via the exchange-coupling force. Consequently, the magnetization that appears in the M layer is oriented to the preset orientation (for example, the "non-A direction") supported by the W layer. As the recording medium returns to room temperature from this condition, the preset orientation is preserved. However, when the compensation temperature is passed in either the M layer or the W layer during the return to room temperature, the orientation of the magnetization in the respective layer is reversed. This process is known as the high temperature process or the high temperature cycle.
The above-described low temperature process and high temperature process may be accomplished with no relation to the magnetic orientation of the M layer and the W layer. In any event, it is desirable for the W layer to be initialized prior to irradiation by the laser beam, thereby making overwriting possible.
With the basic devices, the laser beam is modulated in terms of pulses in accordance with the information to be recorded. However, this process may also be accomplished using conventional magnetooptical recording media, since the means for modulating the beam strength as pulses in accordance with digitized information to be recorded on the recording medium is well known. Such a means is explained, for example, in detail in The Bell System Technical Journal, Vol. 62 (1983), 1923-1936, the entire disclosure of which is incorporated herein by reference. Accordingly, alternative modulations are easily obtainable by adjusting the existing modulation means to the desired necessary high level and low level of the beam strength. One skilled in the art would readily recognize how to make such adjustments to the modulating means if a high level and a low level of the beam strength are provided.
A special characteristic of the basic devices is the high level and low level strength of the laser beam. When the beam strength is at the high level, the "A direction" magnetization of the W layer is reversed to the "non-A direction" by the magnetic recording field Hb or another external means. Through this "non-A direction" magnetization of the W layer, a mark is formed in the M layer having "non-A direction" magnetization (or "A direction" magnetization). When the beam strength is at the low level, the orientation of the W layer magnetization is unchanged from the initialized state, and through the operation of the W layer (this operation being the transfer to the M layer through the exchange-coupling force), a mark is formed in the M layer having an "A direction" magnetization (or "non-A direction" magnetization).
The recording medium used in the basic devices has a multiple layer structure including an M layer and a W layer. The M layer comprises a magnetic layer wherein the coercivity force is high at room temperature and the magnetization reversal temperature is low. The W layer is a magnetic layer wherein, in contrast to the M layer, the coercivity force is weak at room temperature and the magnetization reversal temperature is high. Both the M layer and the W layer can also be comprised of multiple layers themselves. In certain cases, an intermediate layer can exist between the M layer and the W layer. For example, an intermediate layer with an exchange-coupling force of .sigma..sub.w may exist between the M and W layers. Hereinafter, this intermediate layer will be referred to as the I layer. Further information about the I layer can be found by referring to Japanese unexamined patent publications Sho 64-50257 and Hei 1-273248, the disclosures of which are totally incorporated herein by reference.
In addition, a number of references are available with reference to magnetooptical recording with overwrite capabilities, including Japanese unexamined patent publications Hei 4-123339 and Hei 4-134741, which are totally incorporated herein by reference, but further explanation is omitted here. A disk having a four layer structure, which is disclosed in Japanese unexamined patent publication Hei 4-123339, has, in addition to an M layer and a W layer, an initializing layer (hereinafter abbreviated as an Ini layer), and a switching layer (hereinafter abbreviated as a S layer) between the Ini layer and the W layer. The S layer switches the exchangecoupling force between the two layers on and off.
In order to boost the C/N value, a structure for a magnetooptical recording medium has been proposed that also includes a readout layer (hereinafter abbreviated as a R layer) above the M layer (i.e., on the side of the disk on which the laser beam is incident). The R layer should have a higher Curie point and a higher Kerr effect than the M layer. Such a structure is described, for example, in Japanese unexamined patent publications Sho 63-64651 and Sho 63-48637, the entire disclosures of which are incorporated herein by reference. This proposed R layer is also comprised of a RE-TM alloy.
Because recording on a magnetooptical recording medium is usually accomplished using the thermal energy of a laser beam, quite small marks are made by using the center of the beam spot. However, because reproduction is accomplished by optically reading the medium using a narrowed laser beam, it is in principle impossible to accomplish reproduction using a beam spot that is smaller than the refraction limit. At present, given the wavelength of semiconductor lasers that are commercially available, the beam diameter cannot be made smaller than about 1 .mu.m. Consequently it is generally impossible to make the size of the marks much smaller than 1 .mu.m. For this reason, the spacing of marks on the track and the spacing between tracks (track pitch) is restricted.
Accordingly, when either of the restrictions as to mark size and track spacing is exceeded, for example when the spacing between marks on a track is made smaller in order to produce high density recordings, several marks are reproduced simultaneously. In addition, when the spacing between tracks (track pitch) is reduced beyond the size restriction, the result is a high degree of simultaneous reproduction of marks on adjacent tracks, otherwise known as "cross talk." In either case, the quality of the reproduction signal cannot be maintained (the C/N value becomes smaller), and as a result the reproduction of the information cannot be properly accomplished.
These restrictions have placed an effective limit on the recording density of a magnetooptical recording medium. In the case of a magnetooptical recording medium with overwrite capabilities, these restrictions are aggravated because the conditions needed to make overwriting possible also have to be met.